/Engine/Plugins/Runtime/GameplayAbilities/Source/GameplayAbilities/Public/GameplayPrediction.h
The key to understanding this function is that when a key is received by the server, we note which connection gave it to us. We only serialize the key back to that client.
bool NetSerialize ( [FArchive](API\Runtime\Core\Serialization\FArchive) & Ar, class [UPackageMap](API\Runtime\CoreUObject\UObject\UPackageMap) * Map, bool & bOutSuccess )